1. Field of the Invention
The present invention relates to a shielding unit arranged between an end of a substrate and a heat source, when the substrate such as a semiconductor substrate is subjected to a thermal processing. and the present invention relates to a thermal processing apparatus, and a thermal processing method of the same. The present invention relates more particularly to a thermal processing apparatus and a thermal processing method, in which the thermal processing is conducted with a high rising rate of the temperature of the substrate until it reaches to a desired value and excellent uniformity of temperature in a surface of the substrate can be achieved.
2. Description of the Related Art
A thermal processing technology for a semiconductor substrate such as Si and GaAs, and a glass substrate is inevitable on manufacturing of micro electronic devices such as DRAMs, SRAMs. EEPROMs and CCDs and panel display devices such as TFTs formed on glass.
For a thermal processing apparatus employed in such thermal processing, there are two systems: one is a sheet processing system for processing a sheet of a substrate, and the other is a batch system for processing a plurality of substrates simultaneously.
In both systems, it is inevitable to increase a rising rate or temperature per unit time until temperature in the thermal processing apparatus is raised to desired thermal processing temperature, namely, a lamp rate. A temperature distribution in a surface of the substrate to be treated should be uniform.
Since a total processing time for thermal processing can be shortened by virtue of a high lamp rate, a time required to manufacture semiconductor devices will be shortened.
Furthermore, in order to achieve a high integration of the semiconductor devices, a junction depth within the semiconductor devices must be small. Here, the magnitude of the junction depth is determined not only by a thermal processing time at desired temperature but also by the difference between heating values given to the substrate and that released from the substrate for a certain period of time. This certain period of time is from a time until the substrate rises to this desired temperature to a time until the substrate is cooled after thermal processing. In general, the higher the lamp rate, namely, the shorter a time until temperature rises to a desired value, the less the heating value given to the substrate. Therefore, the junction depth within the semiconductor device becomes shallower. In other words, it will be possible to form a shallower junction by increasing the lamp rate.
Furthermore, it is possible to achieve uniformity of characteristics of elements formed in the semiconductor substrate by making a temperature distribution in the substrate uniform, whereby a yield in manufacturing increases.
Furthermore, defects, for example, such as a slippage and a warp are caused by the temperature distribution in the substrate during the thermal processing. Therefore, it is possible to prevent the occurrence of the defects by making the temperature distribution in the substrate uniform, whereby a high yield in manufacturing the semiconductor devices and an improvement of characteristics of the semiconductor devices are achieved.
To achieve the uniformity of the characteristics among the element and to prevent the occurrence of the defects in the substrate, it is important to make the temperature distribution in the surface of the substrate uniform, not only for a period of time of the thermal processing at the desired temperature but also for both periods of time of increasing the temperature of the substrate and decreasing the temperature of the substrate.
However, the increase in the uniformity of the temperature distribution of the substrate is generally contrary to the increase in the lamp rate. Namely, it becomes difficult to achieve uniformity in the temperature distribution in the substrate as the lamp rate increases. The reason for this fact will be described with reference to FIGS. 7(a) and 7(b). FIGS. 7(a) and 7(b) show an apparatus in which heat is radiated into a substrate 1 such as Si. Referring to FIGS. 7(a) and 7(b), the heat radiated into a surface of the substrate 1 is denoted as a symbol s, and the heat radiated perpendicularly into an end of the substrate 1 is denoted as a symbol p.
Particularly, while the temperature of the semiconductor substrate is low (see FIG. 7(a)), the substrate behaves as a transmissible body for the heat radiation, whereby the substrate will hardly absorb the heat. In a crystalline Si substrate of a low impurity concentration of about 1.times.10.sup.15 cm.sup.-3, when the temperature is below 600.degree. C., an absorption length for the heat radiation is, for example, about 10 mm.
Here, a thickness of the Si substrate is, for example, about 1 mm or less, and a diameter of the Si substrate is about several hundreds of millimeters. Specifically, as shown in FIG. 7(a), sincu the absorption length for the heat radiation is longer in comparison with the thickness of the Si substrate, most of the heat S radiated into the surface of the substrate is not absorbed in the substrate, and it transmits through the substrate.
On the other hand, since the absorption length for the heat radiation is shorter in comparison with the diameter of the Si substrate, the heat P radiated into the end of the substrates approximately in perpendicular to the substrate, namely, in a parallel direction with the surface of the substrate, is absorbed in the substrate without transmitting through the substrate.
In the above-described manner, while the average temperatures of the substrate is in a low temperature range, the temperature in the end of the substrate increases at a higher rising rate in comparison with the center of the substrate. Therefore, the temperature in the end of the substrate becomes higher than that in the center of the substrate, whereby a large temperature difference is produced in the surface of the substrate.
When the average temperature of the substrate becomes high see FIG. 7(b)), the substrate shows a non-transmissible property for a heat radiation. The absorption length of the substrate for the heat radiation becomes very small in comparison with the thickness of the substrate. Therefore, if the heat radiation into the substrate from a heater is uniform, the substrate is heated uniformly, whereby the temperature rising rate becomes uniform.
However, the temperature difference between the end of the substrate and the center thereof is produced at the low temperature range. Although such temperature distribution in the substrate is extinguished by thermal conduction, time is needed until the temperature distribution in the substrate changes to be uniform perfectly.
Particularly, during a rapid thermal processing, the time until the substrate is raised to the desired temperature of 1000.degree. C. is, for example, only about 10 seconds. The time until the temperature of the substrate reaches the desired value after it has exceeded the low temperature range is not sufficient enough for the temperature difference to disappear by virtue of the thermal conduction, which was produced in the low temperature range.
For this reason, also in the range where the temperature of the substrate is high, the large temperature difference remains, so that there is a possibility of occurrence of slippages and the like.
The problem of the temperature difference in the surface of the substrate produced in the low temperature range during the above-described thermal processing is especially severe when the heat is radiated uniformly from both directions of the surface of the substrate and the end of the substrate. For example, in a hot wall type RTP (rapid thermal processing) apparatus employing a resistance heater there is a problem because a surface and an end of a substrate are radiated by heat with approximately equal intensity.
A structure of the conventional hot wall type RTP apparatus employing the resistance heater is illustrated in FIG. 8.
The conventional RTP apparatus comprises a substrate holding unit 2 for mounting a substrate 1; a housing 3 for constituting a processing chamber which houses the substrate 1 mounted on the substrate holding unit 2; a resistance heater 4 arranged around the housing 3; and a driving means 5 equipped to the substrate holding unit 2 to move upward and downward the substrate mounted on the substrate holding unit 2.
In the RTP apparatus having the above-described structure, the substrate 1 is mounted on the substrate holding unit 2, and the substrate holding unit 2 is, for example, moved upward by means of the driving means 5, whereby the substrate 1 is brought near the resistance heater 4. Thus, the temperature of the substrate 1 is raised quickly. Furthermore, by holding the substrate 1 close to the resistance heater 4, the temperature of the substrate 1 is kept constant. Furthermore, the substrate 1 is kept away from the resistance heater4 by moving the substrate holding unit 2, for example, downward by means of the driving means 5, whereby the temperature of the substrate 1 is cooled quickly.
The RTP apparatus employing the above described resistance heater has a structure such that the resistance heater 4 surrounds the substrate 1 during the thermal processing. Consequently, the RTP is able to perform the thermal processing for the substrate 1 utilizing thermal energy more effectively, in comparison with, for example, a thermal processing apparatus which heats a substrate using a lamp. In addition, the RTP apparatus has a feature in that the apparatus is able to keep the temperature of the substrate constant stably during the thermal processing.
However, as described above, since the RTP apparatus is constructed such that heat is radiated in the directions toward both the surface of the substrate 1 and the end thereof from the outside approximately uniformly, there is the problem that the temperature difference between the surface of the center and the end thereof exists due to the transmissible property of the substrate for the heat radiation while the temperature of the substrate is raised, especially during the low temperature range.
Furthermore, since the temperature of the substrate is raised rapidly until it reaches the desired temperature, there is a problem that the temperature difference occurred while the temperature of the substrate is in the low temperature range can not be perfectly lessened by the thermal conduction.
In order to solve these problems, a method is considered, wherein in course of raising the temperature of the substrate 1, a rise in the temperature of the substrate 1 is stopped at the temperature of the substrate 1, for example, at about 600 to 700.degree. C., and the temperature of the substrate 1 is kept at this temperature until the temperature difference caused in the low temperature range is perfectly extinguished by the thermal conduction. However, when this method is employed, the time until the temperature of the substrate 1 reaches to the desired value is prolonged, whereby the total time for the thermal processing becomes long.
As described above, the conventional RTP apparatus employing the resistance heater involves inherently a problem of difficulty in increasing a rising rate of the temperature of the substrate as well as in shortening the processing time. In the conventional RTP, there is also a problem of difficulty in achieving uniformity of the temperature in the substrate uniform.